Titania, titanium oxide (IV) or titanium dioxide is a chemical compound having the formula TiO2. Among other things, it is used in photocatalyzed advanced oxidation processes.
Titanium oxide (IV) TiO2, occurs in nature in several ways:
rutile (tetragonal structure),
anatase (tetragonal structure), and
brookite (orthorhombic structure).
Titanium oxide (IV) rutile and titanium oxide (IV) anatase are produced industrially in large quantities and are used as pigments and catalysts, as well as in the production of ceramic materials.
Titanium oxide (IV) is very important as a white pigment for its dispersion properties, chemical stability and non-toxicity. Titanium oxide (IV) is the most important inorganic pigment in terms of world production
The science of metal oxide surfaces is a field of great interest for the catalysis, where surface oxides generally play a very important role, since most metals are oxidized when exposed to this environment. Titania (TiO2) is used in heterogeneous catalysis, as a photocatalyst, in solar cells for producing hydrogen and electrical energy, such as gas sensor, as anticorrosive coating, as optical coatings, in ceramics, in electronic devices and as a white pigment in paints and cosmetics.
TiO2 in its rutile-anatase phases is the most studied in the science of metal oxides. The study of TiO2 in heterogeneous catalysis and the role of metals that are incorporated when acting as the catalyst support, reports its frequent use as a model for other Metal Oxide-metal systems, for all information that can be obtained from titania based catalysts.
One of the effects studied is the titania “Strong Metal Support Interaction (SMSI)”, which is presented by encapsulation of the metal particles by reduction of TiOx overlayer.
A typical catalyst system is TiO2-vanadium, used for selective oxidation reactions, and there is a whole field of research concerning vanadium and the vanadium-TiO2 system, where some studies consider titania not properly as a support, but that its addition there to may substantially modify metal based catalysts.
Other areas of research and development are the photoelectric and photochemical properties of TiO2, where the pioneering work in this area is to Fijishima A., Honda K, Nature 258 (1972) 37 on the photolysis of water to decompose into H2 and O2, where the point is to analyze the role of defect states of TiO2 surface. Regarding the photochemical process to produce solar energy, TiO2 has a low quantum yield, but colloidal suspensions have been used with addition of “dyed molecules” that can improve its use efficiency in solar cells.
A very active field of TiO2 is in the photo-assisted reactions for reduction or degradation of volatile organic compounds (VOCs), the titania is a semiconductor and the hole-electron pair is created through the irradiation of UV light, generating loads that can lead to migrate to the surface where they react with the adsorbed water and oxygen to produce radicals, which interact with organic molecules to carry them to full decomposition to CO2 and H2O. By photo-assisted reactions using TiO2 it is possible to purify water, bacterial disinfection, processes of self-cleaning mirrors and glass, and protective coating (preservation of art works), among the most important.
TiO2 is used as the oxygen sensor to control the air/fuel ratio (A/F) in motor vehicles, which is designed for two types of temperature ranges, for example, at high temperature it is used for detecting oxygen in a long range of partial pressures, doping it with tri and pentavalent ions. For the case of low temperature Pt is added which leads to the formation of a diode type called “Schottky diode”, which makes it highly sensitive against oxygen.
Among the applications of TiO2, in so-called advanced technologies in the field of materials science, is its design as the best dielectric bridge to replace SiO2-MOSFET devices (MOSFET.—Metal Oxide Semiconductor Field Effect Transistor), where the constraint on the use of titania is the dimension in thin SiO2 films, which would be solved by preparing nanostructured materials of titania.
Other significant developments in photo catalytic technology of TiO2, performed in the United States, are glass micro bubbles for specific application in the cleaning of oil films on water, particularly when most of an oil spill was removed. Also, it is the use of TiO2 in photo catalysis for water supplying space crafts, according to the requirements of NASA (NASA.—National Aeronautics and Space Administration of the United States).
In the field of development of new catalytic materials “new generation materials” nanostructured materials are included, which have been the subject of various research and technological developments. The application of these new materials is varied, but in particular, they mostly are targeted to the fields of catalysis and medicine, and in particular the area of environmental catalysis. In the field of environmental catalysis, the development of nanostructured semiconductor materials for use in catalytic reactions is of great interest, such as titania, particularly in the processes of reduction or elimination of contaminants in gas or liquid phase.
The preparation or synthesis methods play an important role in the physicochemical properties of TiO2. In the preparation of the TiO2 various methods have been employed such as hydrolysis of titanium precursors followed by thermal treatment, sol-gel synthesis by pyrolysis, hydrothermal methods, and, in some cases, only by modifying the commercial TiO2, being Degussa P25 of the more used.
According to reports in the literature, in most of the methods, variables that have the greatest impact on the preparation of TiO2 include pH, hydrolyzing agents and hydrolysis temperature, but its effect depends greatly on the synthesis method applied, as for example, in the sol-gel method, variables such as the water/alkoxide ratio and the type of precursor used among others, have a significant impact for attain special physicochemical properties.
Among the most important physicochemical properties of TiO2 as a catalytic material we find its texture, mainly: surface area, volume and pore size. The study of solid texture (size, shape and pore size distribution) is performed by physical adsorption of gas, this method being developed by Brunauer, Emmett and Teller, so called BET method, and its complement the BJH method, developed by Barrett, Joyner and Halenda, for determining pore diameter and volume.
In the study of materials, X-Ray Diffraction (XRD) is considered indispensable. The TiO2 as catalyst support and/or catalyst has three crystal structures: brookite, anatase and rutile. Rutile phase is stable at high temperature (>700 C), the anatase is stable at low temperature (300-600° C.), and the brookite is a transition between the first two. Both the rutile and anatase are tetragonal crystal systems and brookite, usually located in minerals, presents an orthorhombic crystal system, apparently without catalytic properties.
The XRD technique can be applied in both qualitative and quantitative analysis of samples, where through these, it is possible to identify the compounds that make up the sample, and to evaluate the proportion of such compounds and calculate the size of its crystals. The above information can be obtained through the application of basic tools, such as Bragg's Law and the Formula of Integral Intensities. The information you can get is:
Space group and unit cell geometry, obtained from the collection of Bragg angles (2θ); from these values we can also perform a qualitative identification of the crystalline phases;
Crystal size determination, by measuring the peak broadening, with which we also can indicate the crystal purity;
Atomic positions in the unit cell, by measuring the integral intensities of the peaks, which in turn enables the quantitative analysis of the phases present in the sample, and
Analysis of texture, residual stress measurement and phase diagrams.
The determination of the energy band gap or band gap (Eg) of TiO2 is essential for its activity in photo catalytic processes, and is obtained from the UV-vis spectra in the region of 200-800 nm. This region presents the fundamental transition from the valence band to the conduction band
By Fourier Transform Infrared Spectroscopy (FTIR), one can identify the TiO2 functional groups such as the identification of OH groups in its structure, which determines the degree of hydroxylation, important feature of the titania as catalyst material.
By analyzing Transmission Electron Microscopy (TEM) it is possible to determine its morphology, mainly the average crystal size of the TiO2. Also, by selecting a crystal in the different zones of the micrographs, it is possible to obtain the single diffraction pattern, along with their corresponding interplanar spacings with the Digital Micrograph program, which are compared with those of JCPDS classified cards for TiO2 (JCPDS.—Joint Committee on Powder Diffraction Standards), thereby determining the crystal structure in the corresponding direction (hkl).
Nanotechnology in the area of materials covers the fields of: design, creation, synthesis, manipulation and application of nanostructured materials, devices and functional systems through control of matter at the nanoscale.
The material properties can change considerably when their size is reduced to particles in the nanometer scale. In materials science, “particle” is a general term to describe small solid objects with any size ranging from the atomic scale (10−10 m) to the microscopic scale (10−3 m), but the size of the particle is often 10−9 to 10−5 m. Large particles (>10−6 m) are commonly called grains (zeolites, carbons) and small particles (<2 nm) are frequently aggregated (metals) or clusters (metals, oxides). The term “crystallite” describes a small single crystal, the particles may be formed by one or more crystals.
Physical and chemical properties of a material are determined by the type of interactions between the electrons and between ions and electrons. By reducing the space where the electrons can move it is possible that novel effects appear due to the space confinement, this causes modifications on the energy levels in which the electrons can be within the particles. Because of this, and the fact that the surface to volume ratio is greatly increased, the nanoparticles exhibit new properties, which do not appear in the material in large quantities (“in bulk”), nor in the fundamental entities that constitute the solid.
There are two types of nanotechnology to prepare nanostructured materials:
The “Top-Down” method, which refers to the design of nanomaterials with reduced size (larger to smaller), and is based on the mechanisms to obtain nanoscale structures. This type of nanotechnology has been used in different fields, with the field of electronics the most applicable, but recently other fields have been incorporating, such as medicine and the protection of the environment, and
The “Bottom-Up”, which refers to the process of self-assembly, literally from a smaller size to a larger one, starting from a nanometric structure as a molecule passing through a process of self-assembly or assembly to create a mechanism larger than the initial mechanism. This is considered as the only and “real” nano approach, which allows that the material can be controlled in an extremely precise nanoscale.    Some of its properties are:
Increase in the surface area/volume ratio, inducing a huge increase in the interfacial area of the species on the surface;
Changes in the electronic structure of the species of the nanoparticle;
Changes in the ordering (crystal structure, interatomic distances, etc. . . . ) of the species in the nanoparticle and the presence of defects, and
Confinement and quantum size effects due to the confinement of the charge carriers within the nanoparticle.
Among the main patent documents of the state of the art, the inventors identified as the closest to the present invention the following:
In Mexican Patent MX204.757 “Proceso mejorado para la obtención de óxidos de titanio tipo rutilo” (Improved process for producing rutile type titanium oxides) issued on Oct. 16, 2001, Isaac Schifter Secora and Luis Francisco Pedraza Archila recite an improved process for producing titanium oxide (TiO2) rutile, by direct oxidation of titanium trichloride at low temperature, comprising the steps of:
1. preparing a solution of titanium trichloride in water, at a temperature lower than 5° C.;
2. continuously stirring the solution at room temperature, to put in contact with atmospheric air;
3. increasing the temperature of the solution to 30-80° C., and continue stirring for 2 to 30 days, until a white precipitate is obtained;
4. cooling the reaction mixture to room temperature and filter the precipitate consisting of a gel;
5. washing the product with deionized water;
6. filtering and drying the solid at temperatures of 30-130° C., for 10-70 hours, and
7. calcining the solid at temperatures of 200-800° C., in air for 1-20 hours    where:
As starting material titanium trichloride of 99% purity, titanium oxides or alkoxides are used;
Filtration is performed under vacuum;
The titanium trichloride solution is contacted with oxygen, and
The titanium trichloride results from contact of titanium oxide with hydrochloric acid.
Titanium dioxide of high purity, classified according to its crystallographic structure as determined by X-ray diffraction, such as rutile type TiO2, by their physico-chemical properties can be used for application as a catalyst or catalyst support, photoconductive pigment, photocatalyst, especially in photodegradation of chlorinated hydrocarbons (in residual effluents).
In U.S. Pat. No. 6,677,063 B2 “Methods for obtaining photoactive coatings and/or anatase crystalline phase of titanium oxides and articles made therefrom”, published on Jan. 13, 2004, James J. Finley refers to obtaining hydrophilic titanium oxide and/or rutile and anatase by ionic bombarding deposition of titanium metal oxide on a zirconium oxide film in the cubic phase.
Another technique is to deposit the titanium metal on a zinc oxide film in the cubic phase and heating the coating in an oxidizing atmosphere to provide anatase and/or rutile phase(s) of titanium oxide.
In the patent application U.S. 2006/0,091,079 A1 “Methods of preparing the product of titanium oxide surface-active and its use in water treatment processes”, published on May 4, 2006, Meng et al. refer to a method for producing a crystalline titanium oxide with active surface that has a high adsorptive capacity and a high adsorption ratio with respect to dissolved contaminants including the steps of preparing a titanium oxide precipitate from a mixture comprising a hydrolysable titanium compound and heating the precipitate at a temperature below 300° C., without the calcination of the precipitate. Preferably, the titanium oxide product includes crystalline anatase having primary crystallite diameters in the range of 1-30 nm. The product of crystalline titanium oxide with active surface is used in the methods to remove inorganic contaminants dissolved from dilute aqueous streams by suspending the product in an aqueous stream or filtering an aqueous stream through a product bed.
In another method, a hydrolysable titanium compound is added to an aqueous stream so that titanium oxides form a co-precipitate with the pollutants dissolved in a bed of particulate material.
In Canadian patent application CA1,156,210 (A1) “Process of preparation of catalysts or catalyst carriers based on titania and their use in the Claus process of sulfur synthesis”, published on Nov. 1, 1983, Dupin et al. refers to an improved process for preparing catalysts or catalyst carriers based on titania for the Claus process for sulfur synthesis, characterized in that it comprises the following steps:
1) kneading a mixture containing from 1 to 40% by weight water, up to 15% by weight of shaping additive, 45 to 99% by weight of a titanium oxide powder poorly crystallized and/or amorphous that presents a loss on ignition ranging from 1 to 50 wt %;
2) shaping this mixture, and
3) the mixture is dried and then products obtained are calcined at a temperature of from 200 to 900° C.
In the patent application U.S. Pat. No. 6,034,203 A “Catalysis with titanium oxides”, published on Mar. 7, 2000, Lustig et al. refer to a process that can be used in oligomerization, polymerization or depolymerization such as, for example, production of a polyester. The process comprises contacting a carbonyl compound in the presence of a composition, with an alcohol. The catalyst comprises a catalyst having the formula: MxTi(III) Ti(IV)yO(x+3+4y)/2 where M is an alkali metal, Ti(III) is titanium in the oxidation state +3, Ti(IV) is titanium in the +4 oxidation state, x and y are numbers greater than or equal to zero wherein, if x equals zero, y is a number less than ½.
In patent application MY 140,229 (A) “Method for removing iron oxide deposits from the surface of titanium components,” published Dec. 31, 2009, Belmonte et al. refer to a method and solvent composition capable of removing iron oxide deposits from the surface of titanium components without damaging the fundamental titanium component. The iron oxide deposits may be removed from the surface of the titanium component by contacting the titanium component with the solvent composition of the invention. The solvent composition may then be removed from contact with the titanium component to obtain a recyclable solvent composition that is recycled in repeated contact with the titanium component. The solvent composition comprises an aqueous mixture of an organic acid and hydrohalogenic acid.
Previous technologies known to the applicant, were overcome by the present invention, since none of the cited references relate to a semiconductor material of titania nanostructured comprising amorphous crystalline phases: anatase, rutile and brookite, as well as its production process.
It is therefore an object of the present invention to provide a semiconductor material consisting basically of titanium oxide, with the special feature of being nanostructures, which confers special physicochemical properties (textural and morphological) with ability to disperse and stabilize metallic particles with high activity and selectivity in catalytic processes mainly.
Another object of the present invention is to provide a process for producing a titania nanostructured semiconductor material via the sol-gel method.
A further object of the present invention to provide a process for producing a titania nanostructured semiconductor material where the dimension of the crystal size of the titania nanostructures of the semiconductor material depends on the particular handling or set of variables of so-gel method such as the types of metallic alkoxides of titanium used, the characteristics of the solvents, the alkoxide/water ratio, and the medium in which the hydrolysis takes place, which can be acidic or basic.